This disclosure generally relates to apparatus and methods for optimizing the operation of ion thruster arrays.
Ion propulsion generally involves employing an ionized gas accelerated electrically across charged grids to develop thrust. The electrically accelerated particles can achieve very high speeds. The gas used is typically a noble gas, such as xenon. The principal advantage afforded by ion propulsion systems over conventional chemical propulsion systems is their very high efficiency. For example, with the same amount of fuel mass, an ion propulsion system can achieve a final velocity as much as ten times higher than that obtainable with a chemical propulsion system. Although they are efficient, ion propulsion systems develop very low thrust when compared with chemical propulsion systems. This reality has narrowed the range of ion propulsion applications. However, ion propulsion is well suited for space applications where low thrust is often acceptable and fuel efficiency is critical. Many spacecraft, including satellites as well as exploration vehicles, use ion propulsion systems.
For example, spacecraft such as communications satellites now commonly utilize ion propulsion for station keeping and other functions. Ion thrusters utilize electrical power generated by the solar cells of the satellite to supply energy to a propellant to generate the propulsion. In a typical satellite ion thruster, thrust is created by accelerating positive ions through a series of gridded electrodes at one end of a thrust chamber. The electrodes, known as an ion extraction assembly, create thousands of tiny beams of thrust. The beams are prevented from being electrically attracted back to the thruster by an external electron-emitting neutralizer. The power controller is the device which serves to provide electrical control and power to drive the ion thruster, including control of the emission currents in the discharge and neutralizer cathodes.
It is known to use an Xenon Ion Propulsion System (XIPS) to provide thrust for station keeping and transfer orbit of satellites. XIPS uses electricity from the Sun and a working gas to provide propulsion through acceleration of charged ions. A heritage satellite used four ion thrusters (two pairs) and had four separate power processing units (referred to here as “XIPS power controllers” (XPCs)) so that all four thrusters would be capable of being turned on simultaneously. Consequently, this added considerably to the mass required to drive the ion thruster array.
U.S. Pat. No. 6,948,305 disclosed an XIPS comprising a power processing system having reduced mass. That power processing system allowed a single XPC (referred to as a “power processing unit” in that patent) to power a plurality of ion thrusters in an array with the voltage-regulated supplies common to certain elements of the ion thrusters. (The current-regulated supplies have individual outputs so as to provide desired controlled currents to the anodes, keepers and heaters.) The advantage of this approach is mass savings in the voltage-regulated supplies and a significant reduction in the overall packaging mass.
It is further known to equip a satellite with two redundant subsystems, each subsystem comprising one XPC wired to two ion thrusters. The two subsystems are completely independent, but both subsystems can be rendered inoperative if, for example, one subsystem has a thruster problem and the other subsystem has a XPC problem. To address this problem, an XIPS Relay Unit (XRU) was added between the ion thrusters and the XPC. This XRU allowed the ion thrusters and the XPC of the other redundant subsystem to be used. As a result, either XPC could fire any one of four ion thrusters. The addition of the XRU allowed for multiple failures in the XPC and thrusters, while maintaining a working subsystem. All that was required to maintain spacecraft control was thrust from one thruster. Each XRU consisted of a relay bank operated by a relay driver circuit. The ion thrusters and XPC were wired to give maximum redundancy. A known XRU design utilizes up to 36 relays to perform the switching function.
In the event that one of the relays fails to switch properly or is in the wrong configuration, it is possible that all four thrusters will be energized with XPC power. If any one of the four thrusters has an electrical short, then no thruster can be fired. The existing solution is to use daughter relays to indicate the commands were properly sent and a signal pulse was received by the critical relays. In addition, the first turn-on of the ion thruster would determine if a fault has occurred. In addition, one XRU design utilizes redundant relays to protect against a single relay fault on critical circuits.
In the event that a relay fault has occurred, it would be desirable to take corrective action prior to the start of the XIPS operation. There is a need for means for enabling an XIPS to test the critical relay configuration prior to operation of the XIPS so that corrective action can be taken.